A new type of microscopic particle, which has a hard shell and a soft core, that changes structure depending on its temperature has been developed by Walter Richtering and Ingo Berndt of the University of Aachen, Germany, and Jan Skov Pedersen of the University of Århus, Denmark.¹  The temperature-sensitive particles could have industrial and biomedical applications as capsules for the controlled release of pharmaceuticals and other agents. 

The particles are polymeric microspheres with a core composed of poly-N-isopropylacrylamide and a poly-N-isopropylmethacrylamide shell. Both polymers swell in water, forming a microgel. However, because of the differing polymer building blocks used to make the shell and core, each polymer differs in how much water it can absorb. This means that the microspheres are densely packed when they are first made, at a temperature of 70ºC, but when they are cooled to 25ºC their structure changes so that the core and shell have the highest water content and the lowest density, and dissolved molecules can pass through the shell into the core, where they disperse. Warming to body temperature (about 39ºC), however, causes only the shell and not the core to swell. This expels the trapped water, making the shell shrink and become denser than the core. Any substances dissolved in the core can no longer pass through the shell and so are locked inside.

As well as their sensitivity to temperature, such polymeric microgels with core-shell architectures have other potential uses. By choosing appropriate components and reaction conditions, the researchers suggest they could construct particles with multiple shells that might be used to trap different compounds in different layers and then allow them to mix at a specific temperature. Such a layered capsule could be used as a microscopic reaction vessel with mixing occurring only at specific temperatures.